The invention concerns the field of certifying, or authenticating data files.
In certain fields, it is indispensable to certify documents to ensure that their contents cannot be contested. When the carrier of the document is paper, it is particularly easy to append a signature to each one of the pages of the said document, and to complete it, if necessary, by an appropriate marking as, for example, by punching.
However, this type of certification can be reproduced by an ill-intentioned person wishing to alter the contents of at least one part of the document concerned.
Moreover, the filing of documents certified on a paper carrier requires considerable filing space and renders their manipulation difficult.
Finally, the making of a certified copy conforming to an original certified document, or of an extract therefrom, requires many operations which increase the price and the time for obtaining it.
Thanks to recent developments in electronics, and more precisely in electronic information technology, it is henceforth possible to store the data of a document in a resident-type memory (a hard disk) or a removable memory (a 31/2 inch-type disk or a 51/4 inch type disk).
However, while it is possible to store the data of a document in a memory-type carrier, it is, on the other hand, not possible to store a physical marking, as, for example, a punch mark or a stamp in relief. In other words, it is possible for an ill intentioned person to alter the stored data of a document, without it being possible for this to be noticed, and then to destroy the original.
To remedy this major drawback, the expert has developed complex algorithms, as for example Bosset's algorithm. These algorithms are set out in detail, particular in the following works:
B. Schneier, Applied Cryptography, 1996, John Wiley & Sons, and PA1 J. Bosset, "Contre les risques d'alteration, un systeme de certification des informations" (Against the risks of alterations, an information certifying system), 01 Informati-que, February 1977. PA1 D. E. Knuth, "The art of computer programming", vol.2, p.32-33, 1981, Addison-Wesley, and PA1 Carter Bays and S. D. Durham, ACM Trans. Math. Software, chapter 1, pp.69-64, 1976. PA1 a memory termed a source memory, of the read-write memory type, and capable of storing words, each accompanied by an extraction status, PA1 a buffer memory arranged to dispose the data of the file in its free spaces, in the form of blocks, and PA1 processing means comprising:
Bosset's algorithm operates on the non-commutative principle of matrix products, which makes it possible to effect a transformation of the data bits of a file stored in a source memory, which can only be inverted with difficulty.
Moreover, Knuth has developed a method aiming to improve the quality of a pseudorandom code generator. This method lies in inserting words into a source memory, and each time a bit is drawn from the memory in a random manner, it is replaced by its transform which is calculated by many operations. This method is set out in detail in the following works:
Moreover, algorithms have been developed which are derived from a technique termed "rucksack" which lies in adding predetermined integers termed "increments", drawn in a pseudorandom manner from an increment memory, in the registers of a memory termed "rucksack".
These algorithms require, moreover, a source memory whose predetermined and fixed content serves to place during each processing of a file, a secret key intended to place the encryptation function into a parametric mode. The data bits of the file are not processed continuously, but in blocks, which permits various possible combinations with the same final result. In spite of the complexity of the processing operations undertaken, it is always possible to decode the file and to gain access to the secret key. In these conditions, it becomes possible to modify the data and then to encode them again.
Besides, irrespective of the efficiency of a cryptographic algorithm, the latter only provides an encrypted transform of the original file, and not a signature of the latter.